Detecting microplastics in organic-rich materials and their potential risks to earthworms in agroecosystems

Microplastics (MPs) are a major emerging contaminant in agroecosystems, due to their significant resistance to degradation in terrestrial environments. Although previous investigations have reported the harmful effects of MPs contamination on soil biological properties, still little is known about the characteristics and fate of MPs in biosolid-amended soils and their risks to soil biota, particularly earthworms. We determined microplastics' concentration, size distribution, and chemical composition in 3 sewage sludge biosolids and 6 biosolid-amended agricultural soils. In addition, we assessed the potential short-term risks of MPs to earthworms' (Amynthas Gracilis and Eisenia Fetida) survival rate and fitness in an environmentally relevant exposure study (28 days). Biosolid-amended soils (1000-3100 MPs kg(-1) dry mass) showed approximate to 30 times lower MPs content than investigated biosolids (55400-73800 MPs kg(-1) dry mass), with microplastic fragment to fibre ratios between 0.2 and 0.6 and 0.3-0.4 in soils and biosolids, respectively. Total MPs dry mass was also approximate to 19 times lower in assessed soils (12-26 mg kg(-1)) than biosolids (328-440 mg kg(-1)). On average 77% and 80% of plastic fragments had a lower dimension than 500 mu m, while 50% and 67% of plastic fibres had a length of less than 1000 mu m in soil and biosolid samples, respectively. Polyethylene (23.6%) was the major source of microplastic contamination in biosolid-amended soils, while polyethylene terephthalate (41.6%) showed the highest concentration in biosolid samples. Spiked polyethylene MPs did not show any significant effect on earthworms' survival rate (93-99%). However, biosolid application significantly (P < 0.05) decreased survival rate of Eisenia Fetida (81%) but showed no significant effect on Amynthas Gracilis (93%). Biosolid amendment significantly (P < 0.05) decreased earthworms' growth rate, with higher impact on Eisenia Fetida than Amynthas Gracilis, while there were no significant differences between control and microplastic spiked treatments. The overall decrease in MPs con-centration of earthworm casts, compared with initial MPs concentrations in soil, indicated that the investigated species did not bioaccumulate MPs during the exposure experiment.

Automated summary

Microplastics are a significant emerging contaminant in agroecosystems and have harmful effects on soil biological properties. The characteristics, fate, and risks of microplastics in biosolid-amended soils, particularly to earthworms, are not well understood. This study investigated the concentration, size distribution, and chemical composition of microplastics in sewage sludge biosolids and biosolid-amended agricultural soils. The results showed lower microplastic content in biosolid-amended soils compared to the investigated biosolids, with polyethylene as the major source. The application of biosolids decreased earthworms' survival rate and growth rate, but microplastic spiked treatments did not show significant differences compared to the control group.